Wax coating process for corrugated paperboard

ABSTRACT

WAX COATED CORRUGATED PAPERBOARD OF IMPROVED WATER RESISTANCE AND FIBER TEAR PROPERTIES IS PRODUCED BY DIPPINH PARTIALLY HYDROATED CORRUGATED PAPERBOARD INTO MOLTEN WAX AT A TEMPERATURE BETWEEN THE MELTING POINT OF THE WAX AND THE BOILING POINT OF WATER, AND THEN DRAINING THE DIPPED PAPERBOARD AT A TEMPERATURE BETWEEN THE MELTING POINT OF THE WAX AND THE BOILING POINT OF WAX. THE TIME AND TEMPERATURE OF THE DIPPING AND DRAINING STEPS ARE CONTROLLED AND CORRELATED WITH THE INITIAL WATER CONTENT OF THE PAPERBOARD AND THE VISOCISITY OF THE MOLTEN WAX SO AS TO ACHEIVE MAXIMUM SURFACE COATING WITH MINIMUM WAX PENETRATION INTO THE INTERIOR OF THE PAPERBOARD ELEMENTS.

States US. Cl. 117-102 R 4 Claims ABSTRACT OF THE DISCLGSURE Wax coated corrugated paperboard of improved water resistance and fiber tear properties is produced by dipping partially hydrated corrugated paperboard into molten wax at a temperature between the melting point of the wax and the boiling point of water, and then draining the dipped paperboard at a temperature between the melting point of the wax and the boiling point of water. The time and temperature of the dipping and draining steps are controlled and correlated with the initial water content of the paperboard and the viscosity of the molten wax so as to achieve maximum surface coating with minimum wax penetration into the interior of the paperboard elements.

amma.

RELATED APPLICATIONS THE INVENTION This invention relates to a new and improved wax coating process and to wax coated corrugated paperboard produced by this process.

Wax impregnated corrugated paperboard finds considerable use as containers for shipping fruits, vegetables and other perishables, the containers being placed in contact with ice to keep the contents cold and fresh.

To maintain low water pickup of the cartons due to the melting ice, the paperboards are dipped in wax at a temperature sufficient to thoroughly impregnate them with a large amount of wax. To assure satisfactory impregnation, the dipping operation is carried out at about 240 F. Following impregnation, the wax is drained from the paperboard at about 240 F. and then allowed to cool and harden.

It has now been found that the foregoing conventional procedure is disadvantageous from several standpoints. Firstly, the high temperatures employed tend to drive essentially all of the adsorbed water from the paperboard structure, and it is well known that the bending quality or stretch of a paper product is a function of the moisture content thereof, the paper becoming more brittle and rigid at low moisture contents. As a result, the high temperature-impregnated paperboard tends to crack or break along fold lines when box blanks are later scored and folded into a box shell. Secondly, it has been found that dipping and draining the paperboard at high temperatures, entailing a low wax viscosity, results in some of the outer fibers protruding from the paperboard surface being inadequately coated thus providing entry ports for water, resulting in reduced wet strength. Finally, it has been found that the conventional hot-dip procedure results in a wasteful penetration of the wax into the interior of the paperboard liners and the corrugated medium, which serves no useful purpose if the interior portions are adequately hydrated to maintain flexibility and tear strength; good water resistance requires merely a thorough coating of the fibrous paper surfaces. Thus, the prior art proceatent dures not only are wasteful of wax, but fail to give optimum water resistance and wet strength.

It is therefore, an object of the invention to provide a process for reducing the amount of wax required to effect the same water resistance of a corrugated paperboard carton or to improve the water resistance with the same amount of wax.

Another object is to improve the wet strength and fiber tear properties of a wax impregnated carton.

Another object is to reduce the heating requirements for the wax dipping and drain operations.

Another object is to provide a wax impregnated paperboard which may be used for cartons in which the surface fibers of the carton have been coated with wax.

These and other objects will become apparent from the description to follow.

The objects of the invention are attained by dipping the corrugated board, having an initial water content of at least about 2 weight-percent, and preferably between about 2 and 10 weight-percent, into wax at a low temperature to coat the board, and then draining the wax from the board at a low temperature and for a time which permits the wax to remain on the board surface in suflicient amount to coat the fibers protruding from the board surface. It has been discovered that these protruding fibers, if left uncoated, absorb water readily and convey the water into the interior of the board, thereby decreasing the insulating and the wet strength properties of the board. However, if the wax coating process is performed according to the present invention the fibers protruding from the board surface will also be coated.

Accordingly, the invention comprises impregnating the paperboard by dipping it into wax at a lower wax temperature than normal, and then draining the wax from the board also at a lower temperature than normal. This procedure provides a paperboard in which the major portion of the wax is concentrated near the surface, and where a minor portion of the wax coat the surface of the board and the protruding surface fibers. This wax coated paperboard can exhibit a 65 percent reduction in water uptake if the total amount of wax pickup is the same as in normal procedures. Alternatively, if the same water uptake is desired, the amount of wax required can be decreased by as much as 14 percent to produce this result.

Several factors which influence the dip and drain temperatures include the weight and type of corrugated paperboard employed, the moisture content of the board, the particular wax employed, the specific equipment used in the dipping step, the residence times of the paperboard in the dipping and draining steps, and the amount of wax permitted to drain off the board surface.

A typical corrugated paperboard normally comprises a light weight, corrugated, relatively uncalendered and unfilled kraft paper medium adherently interposed between two relatively light weight sheets of lightly calendered kraft paperboard liners. It will be understood that the thickness of the various individual elements can be selected to impart the necessary strength to the corrugated paperboard products. The liners of single-wall corrugated paperboards are often of different weights, the heavier liner usually being placed on the interior of the box. In general, the minimum paper weight employed in corrugated paperboard for water resistant box construction is 26 pounds per thousand square feet. The combined weight of liners for two to three-walled paperboard usually ranges between about 52 and 264 pounds per thousand square feet of corrugated paperboard, and between 52 and pounds per thousand feet for single wall paperboard. The total thickness of the corrugated structure can vary from approximately -inch to about ;-inch for heavy triple-wall construction.

The process of this invention is designed particularly for use in connection with corrugated paperboards wherein the liners and the medium are composed of uncalendered or only lightly calendered, and relatively unfilled kraft paper, which is relatively porous and exhibits a somewhat furry exterior surface of protruding fibers. The porous nature of these liners and the corrugated medium, taken in conjunction with the geometric complexity of the overall paperboard structure, with its attendant capillary crevices on either side of the glue-junctions of the liners with flutes of the medium, present entirely different problems in respect to optimum wax utilization than those which arise in connection with the coating or impregnation of single-ply paper or paperboards. In the case of single-ply papers or paperboards, excess molten wax drains readily from the plane surfaces thereof, and the temperature and viscosity of the wax melt is hence not such a critical factor.

However, in the dip coating of corrugated paperboard it has been assumed, dirt to the complexity of the inner fluted structure, that adequate drainage of excess wax would require high temperatures and low viscosities. We have found however that this procedure results in a wasteful capillary absorption of wax into the interior of the paperboard elements. The hot draining, assumed to be necessary to achieve maximum wax economy, actually results in draining the wax preferentially from the very areas where it is most needed, namely on the exterior surfaces of the paperboard elements, leaving exposed fibers through which water can enter. The wax absorbed into the interior of the paper structures serves little or no useful purpose.

We have most unexpectedly discovered that a more economical utilization of the wax is obtained by dipping and draining the corrugated paperboard at relatively low temperatures, entailing relatively high viscosities. Notwithstanding the decreased surface drainage which thereby occurs, the much decreased internal absorption of wax into the paper structures (due to the higher viscosity thereof), and the greater elfectiveness of the resulting surface coatings, result in a more eflicient overall wax utilization than is obtained in the conventional hot-dip and drain methods. Even though by our method, somewhat more than a desirable amount of wax may remain deposited by capillary adhesion at the junctions of the flutes with the liners, this disadvantage is overcome by decreased internal absorption and increased effectiveness of the external surface coatings.

To achieve the objectives of our invention, the primary requirement is to maintain the temperature of the wax composition during the dip and drain cycles at below the boiling point of water, and further adjusted to provide a kinematic viscosity between about 10 and 50, preferably about 12-30, centistokes. Further, where wax mixtures are employed containing polymers such as ethylene-vinyl acetate copolymers, the temperature should be above the cloud point thereof.

The paperboard blanks are dipped, normally with the flutes in a substantially vertical alignment, into the melt whereby the wax rises through the internal fluted structure displacing air therefrom, and coats all surfaces of liners and corrugated medium. Suitable dip times may range between about 3 aud 20 seconds. The dip time is further adjusted and correlated with the wax temperature so as to leave at least about 2 weight-percent, and preferably at least 5 Weight-percent, of water in the paperboard. The paperboard is then removed from the molten wax, and allowed to drain in a position such that the wax drains freey from the flute channels defined by the corrugated medium. For this purpose it is ordinarily preferred to support the paperboard with the flutes in substantially a vertical position. Draining times may range between about 30 seconds and minutes, preferably between about 1 and 5 minutes. Proper drain times can be evaluated in terms of the malachite green stain rating test described hereinafter; the stain rating should be greater than 3 but less than 5, preferably between about 3.75 and 4.75.

4 EXAMPLE A wax composition containing Aristowax /134: 96.95%; Elvax 460: 3.0%; and inhibitor: 0.05% was employed as the coating wax. Elvax 460 is the trademark name of an ethylene-vinyl acetate copolymer containing 17-49% vinyl acetate and is sold by E. I. du Pont de Nemours and Company. Aristowax 130/134 is the trademark of a fully refined parafiin wax, having a melting point of about 132 F, average molecular weight 371, and containing about 90 percent straight chain hydrocarbons which is sold by the Union Oil Company of California. The kinematic viscosity characteristics of this blend were as follows:

Temperature, F. Viscosity, centistokes Heavy Duty Glacier Pak unwaxed stock having a water content of about 7 weight-percent, code 37-40248, sold by Georgia Pacific Corporation, Container Division was cut into 8" x 8 squares for testing. The term Heavy Duty refers to C-flute 6933-69 weight, 375 pound Mullen test board used for relatively severe service conditions. Glacier Pal: is Georgia Pacifics trademark for Wax impregnated cartons for top iced shipments of vegetables, fruits and other perishables. The dip and drain cycle was as follows:

(1) The boards were dipped into the wax vertically for one minute: The Wax temperature was 240 F.

(2) The boards were then drained by rotating 115 and holding 25 to the horizontal for one minute.

(3) The boards were then rotated 65 and held vertically for a four minute drain at 240 F.

(4) The boards were finally removed from the drain oven, rotated 180 and cooled at room temperature.

The temperatures employed in the above test are those currently in use for manufacturing wax impregnated cartons for top iced shipments of fruits, vegetables and other erishables.

The above tests were carried out in a small size test apparatus. If a scale-up is desired, a forced air circulation system might be necessary to achieve the desired results.

Wax uptake was evaluated by the formula:

Wax uptake, wt. percent wt. waxed-wt. unwaxed Wt. unwaxed Water uptake was evaluated by the formula:

Water uptake, Wt. percent wt. soaked-wt. unsoaked Wt. unsoaked The water uptake test involves soaking the specimen in water for one hour at 73 F.

Surface Wax is evaluated as follows: Four 1" x 6" strips are cut from two 8" x 8" boards which have been dipped and drained. Two 1" x 6" strips are taken from each board and are cut, one from each edge parallel to the flutes and the direction of dip.

Then approximately one-fourth of the surface, on the heavy liner, is brushed with a stain solution of 24% Water, 74% n-propanol and 2% malachite green. The stain solution is then rinsed oil with water, or wiped oil with a rag and the resulting stain area noted. A whole number is given to each stain according to the following scale. The average of the four sample stains is reported.

ployed in a carton to obtain equivalent compression strength. Alternatively, the same amount of wax can be used to provide significantly greater water resistance and a. correspondingly greater wet column compression strength. Obviously, paperboard having uses other than in cartons can benefit from the coating process of the present invention.

We claim:

1. A method for coating corrugated paperboard with a parafiin wax composition which comprises:

(1) dipping corrugated paperboard having an adsorbed water content of about 2 weight-percent into the molten wax composition at a melt temperature which is (a) below the boiling point of water, and (b) adjusted to provide a kinematic viscosity of the melt between about 12 and 30 centistokes;

(2) removing said paperboard from the molten wax composition before the paperboard water content falls below about 2 Weight-percent;

(3) draining the coated paperboard in a position which allows wax to drain freely from the flute channels defined by the corrugated medium and while maintaining the paperboard at a temperature within the range prescribed in step (1) for a sufiicient time to leave an exterior liner surface having a malachite green stain rating greater than 3 but less than 5; and

(4) cooling the drained paperboard to below the melting point of the wax composition.

2. The method as defined in claim 1 wherein said wax composition contains a minor proportion of a wax-soluble polymer to increase its viscosity, and wherein the temperatures maintained in steps (1) and (3) exceed the crystalline cloud point of the wax-polymer mixture.

3. A method as defined in claim 2 wherein said polymer is selected from the class consisting of: ethylenevinyl acetate copolyrners, ethylene-lower alkyl acrylate copolyrners, vinyl stearate-ethylene copolymers, ethyleneisobutylene copolymers and ethylene-methyl methacrylate copolymers.

4. A method as defined in claim 1 wherein steps (1) and (3) are carried out at temperatures between about and 200 F.

References Cited UNITED STATES PATENTS 3,061,456 10/1962 Davis et a1. 117l58 X 3,177,091 4/1965 Case et a1. ll7l58 X WILLIAM D. MARTIN, Primary Examiner M. R. LUSIGNAN, Assistant Examiner 

